News Release Number: STScI-2004-10

March 4, 2004: This image resembling Vincent van Gogh's painting, "Starry Night," is
Hubble's latest view of an expanding halo of light around a distant star,
named V838 Monocerotis (V838 Mon). This Hubble image was obtained with
the Advanced Camera for Surveys on February 8, 2004. The illumination
of interstellar dust comes from the red supergiant star at the middle
of the image, which gave off a flashbulb-like pulse of light two years
ago. V838 Mon is located about 20,000 light-years away from Earth in
the direction of the constellation Monoceros, placing the star at the
outer edge of our Milky Way galaxy.

Q & A: Understanding the Discovery

1.
What is a light echo?

It is light from a stellar explosion echoing off dust surrounding the
star. V838 Monocerotis produced enough energy in a brief flash to
illuminate surrounding dust, like a spelunker taking a flash picture of
the walls of an undiscovered cavern. The star presumably ejected the
illuminated dust shells in previous outbursts. Light from the latest
outburst travels to the dust and then is reflected to Earth. Because of
this indirect path, the light arrives at Earth months after light from
the star that traveled directly toward Earth.

2.
Why did the star produce this outburst?

Astronomers do not fully understand the star's outburst. It was somewhat
similar to that of a nova, a more common stellar outburst. A typical
nova is a normal star that dumps hydrogen onto a compact white-dwarf
companion star. The hydrogen piles up until it spontaneously explodes by
nuclear fusion  like a titanic hydrogen bomb. This exposes a searing
stellar core, which has a temperature of hundreds of thousands of
degrees Fahrenheit.

By contrast, V838 Monocerotis did not expel its outer layers. Instead,
it grew enormously in size. Its surface temperature dropped to
temperatures that were not much hotter than a light bulb. This behavior
of ballooning to an immense size, but not losing its outer layers, is
very unusual and completely unlike an ordinary nova explosion.

The outburst may represent a transitory stage in a star's evolution
that is rarely seen. The star has some similarities to highly unstable
aging stars called eruptive variables, which suddenly and unpredictably
increase in brightness.